Bimaps allow clients to find not just a value given a key, but to find a
key given a value. Inspired by com.google.guava.bimap. Implemented
here using functors and two maps in two ways: as a class and as a module.
Also supports multi-bi-map, in which a key is mapped to multiple values
as in a multi-map and the inverse view is supported as well under which
any one of the values will provide the key.

Each method ensures us that both maps are updated properly, alleviating
the user or client code from performing the correct operations on both
maps when interacting with either one of them.

See the unit test under src/test for examples of usage.

                            Single values Map     v   Multi-map 
Implemented as a module | Bimap_single_module.ml  | Bimap_multi_module.ml
 Implemented as a class | Bimap_single_class.ml   | Bimap_multi_class.ml